(a) Field of the Invention
The present invention relates to a light emitting display, a display panel, and a driving method thereof. More specifically, the present invention relates to an organic electroluminescent (EL) display.
(b) Description of the Related Art
In general, an organic EL display electrically excites a phosphorous organic compound to emit light, and it voltage- or current-drives N×M organic emitting cells to display images. As shown in FIG. 1, the organic emitting cell includes an anode of indium tin oxide (ITO), an organic thin film, and a cathode layer of metal. The organic thin film has a multi-layer structure including an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL) for maintaining balance between electrons and holes and improving emitting efficiencies, and it further includes an electron injecting layer (EIL) and a hole injecting layer (HIL).
Methods for driving the organic emitting cells include the passive matrix method, and the active matrix method using thin film transistors (TFTs) or metal oxide semiconductor field effect transistors (MOSFETs). The passive matrix method forms cathodes and anodes to cross with each other, and selectively drives lines. The active matrix method connects a TFT and a capacitor with each ITO pixel electrode to thereby maintain a predetermined voltage according to capacitance. The active matrix method is classified as a voltage programming method or a current programming method according to signal forms supplied for maintaining a voltage at a capacitor.
Referring to FIGS. 2 and 3, conventional organic EL displays of the voltage programming and current programming methods will be described.
FIG. 2 shows a conventional voltage programming type pixel circuit for driving an organic EL element, representing one of N×M pixels. Referring to FIG. 2, transistor M1 is coupled to an organic EL element (referred to as an OLED hereinafter) to thus supply current for light emission. The current of transistor M1 is controlled by a data voltage applied through switching transistor M2. In this instance, capacitor C1 for maintaining the applied voltage for a predetermined period is coupled between a source and a gate of transistor M1. Scan line Sn is coupled to a gate of transistor M2, and data line Dm is coupled to a source thereof.
As to an operation of the above-configured pixel, when transistor M2 is turned on according to a select signal applied to the gate of switching transistor M2, a data voltage from data line Dm is applied to the gate of transistor M1. Accordingly, current IOLED flows to transistor M2 in correspondence to a voltage VGS charged between the gate and the source by capacitor C1, and the OLED emits light in correspondence to current IOLED.
In this instance, the current that flows to the OLED is given in Equation 1.IOLED=β/2(VGS−VTH)2=β/2(VDD−VDATA−|VTH|)2  Equation 1                where IOLED is the current flowing to the OLED, VGS is a voltage between the source and the gate of transistor M1, VTH is a threshold voltage at transistor M1, and β is a constant.        
As given in Equation 1, the current corresponding to the applied data voltage is supplied to the OLED, and the OLED gives light in correspondence to the supplied current, according to the pixel circuit of FIG. 2. In this instance, the applied data voltage has multi-stage values within a predetermined range so as to represent gray.
However, the conventional pixel circuit following the voltage programming method has a problem in that it is difficult to obtain high gray because of deviation of a threshold voltage VTH of a TFT and deviations of electron mobility caused by non-uniformity of an assembly process. For example, in the case of driving a TFT of a pixel through 3 volts (3V), voltages are to be supplied to the gate of the TFT for each interval of 12 mV (=3V/256) so as to represent 8-bit (256) grays, and if the threshold voltage of the TFT caused by the non-uniformity of the assembly process deviates, it is difficult to represent high gray. Also, since the value β in Equation 1 changes because of the deviation of the mobility, it becomes even more difficult to represent the high gray.
On assuming that the current source for supplying the current to the pixel circuit is uniform over the whole panel, the pixel circuit of the current programming method can achieve uniform display features even though a driving transistor in each pixel has non-uniform voltage-current characteristics.
FIG. 3 shows a pixel circuit of a conventional current programming method for driving the OLED, representing one of N×M pixels. Referring to FIG. 3, transistor M1 is coupled to the OLED to supply the current for light emission, and the current of transistor M1 is controlled by the data current applied through transistor M2.
First, when transistors M2 and M3 are turned on because of the select signal from scan line Sn, transistor M1 becomes diode-connected, and the voltage matched with data current IDATA from data line Dm is stored in capacitor C1. Next, the select signal from scan line Sn becomes high-level to turn on transistor M4. Then, the power is supplied from power supply voltage VDD, and the current matched with the voltage stored in capacitor C1 flows to the OLED to emit light. In this instance, the current flowing to the OLED is as follows.IOLED=β/2(VGS−VTH)2=IDATA  Equation 2
where VGS is a voltage between the source and the gate of transistor M1, VTH is a threshold voltage at transistor M1, and β is a constant.
As given in Equation 2, since current IOLED flowing to the OLED is the same as data current IDATA in the conventional current pixel circuit, uniform characteristics can be obtained when the programming current source is set to be uniform over the whole panel. However, since current IOLED flowing to the OLED is a fine current, control over the pixel circuit by fine current IDATA problematically requires much time to charge the data line. For example, assuming that the load capacitance of the data line is 30 pF, it requires several milliseconds of time to charge the load of the data line with the data current of several tens to hundreds of nA. This causes a problem that the charging time is not sufficient in consideration of the line time of several tens of microseconds.